[Bugfix] Fix `MQLLMEngine` hanging (#9973)

Signed-off-by: rshaw@neuralmagic.com <rshaw@neuralmagic.com>

vllm/engine/multiprocessing/client.py

@@ -112,7 +112,11 @@ class MQLLMEngineClient(EngineClient):
 
         # Stream for each individual request.
         self.output_queues: Dict[str, asyncio.Queue] = {}
-        self.output_loop = asyncio.create_task(self.run_output_handler_loop())
+
+        # Loop to handle output of the LLMEngine periodically.
+        # Started after the MQLLMEngine is ready so that we can
+        # build the Client in an executor to enable clean shutdown.
+        self.output_loop: Optional[asyncio.Task] = None
 
         # Loop to check health of the LLMEngine periodically.
         # Started after the MQLLMEngine is ready.
@@ -247,6 +251,9 @@ class MQLLMEngineClient(EngineClient):
     async def setup(self):
         """Setup the client before it starts sending server requests."""
 
+        # Start output_loop
+        self.output_loop = asyncio.create_task(self.run_output_handler_loop())
+
         with self.get_data_socket() as socket:
             # Wait until server is ready.
             response = await self._wait_for_server_rpc(socket)
@@ -265,6 +272,7 @@ class MQLLMEngineClient(EngineClient):
         # Cancel background tasks.
         if self.health_loop is not None:
             self.health_loop.cancel()
+        if self.output_loop is not None:
             self.output_loop.cancel()
 
     def _set_errored(self, e: BaseException):

vllm/engine/multiprocessing/engine.py

@@ -349,16 +349,22 @@ class MQLLMEngine:
             self.engine.model_executor._run_workers("stop_profile")
 
 
-def run_mp_engine(engine_args: AsyncEngineArgs, usage_context: UsageContext,
-                  ipc_path: str):
-
-    def signal_handler(*_) -> None:
-        # Interrupt server on sigterm
+def signal_handler(*_) -> None:
     raise KeyboardInterrupt("MQLLMEngine terminated")
 
-    signal.signal(signal.SIGTERM, signal_handler)
 
+def run_mp_engine(engine_args: AsyncEngineArgs, usage_context: UsageContext,
+                  ipc_path: str, engine_alive):
+    try:
         engine = MQLLMEngine.from_engine_args(engine_args=engine_args,
                                               usage_context=usage_context,
                                               ipc_path=ipc_path)
+
+        signal.signal(signal.SIGTERM, signal_handler)
+
         engine.start()
+
+    except BaseException as e:
+        logger.exception(e)
+        engine_alive.value = False
+        raise e

vllm/entrypoints/openai/api_server.py

@@ -171,39 +171,44 @@ async def build_async_engine_client_from_engine_args(
         # so we need to spawn a new process
         context = multiprocessing.get_context("spawn")
 
+        # The Process can raise an exception during startup, which may
+        # not actually result in an exitcode being reported. As a result
+        # we use a shared variable to communicate the information.
+        engine_alive = multiprocessing.Value('b', True, lock=False)
         engine_process = context.Process(target=run_mp_engine,
                                          args=(engine_args,
                                                UsageContext.OPENAI_API_SERVER,
-                                               ipc_path))
+                                               ipc_path, engine_alive))
         engine_process.start()
         engine_pid = engine_process.pid
-        assert engine_pid is not None, "Engine process failed to start"
+        assert engine_pid is not None, "Engine process failed to start."
         logger.info("Started engine process with PID %d", engine_pid)
 
         # Build RPCClient, which conforms to EngineClient Protocol.
-        # NOTE: Actually, this is not true yet. We still need to support
-        # embedding models via RPC (see TODO above)
         engine_config = engine_args.create_engine_config()
-        mp_engine_client = MQLLMEngineClient(ipc_path, engine_config,
+        build_client = partial(MQLLMEngineClient, ipc_path, engine_config,
                                engine_pid)
-
+        mq_engine_client = await asyncio.get_running_loop().run_in_executor(
+            None, build_client)
         try:
             while True:
                 try:
-                    await mp_engine_client.setup()
+                    await mq_engine_client.setup()
                     break
                 except TimeoutError:
-                    if not engine_process.is_alive():
+                    if (not engine_process.is_alive()
+                            or not engine_alive.value):
                         raise RuntimeError(
-                            "Engine process failed to start") from None
+                            "Engine process failed to start. See stack "
+                            "trace for the root cause.") from None
 
-            yield mp_engine_client  # type: ignore[misc]
+            yield mq_engine_client  # type: ignore[misc]
         finally:
             # Ensure rpc server process was terminated
             engine_process.terminate()
 
             # Close all open connections to the backend
-            mp_engine_client.close()
+            mq_engine_client.close()
 
             # Wait for engine process to join
             engine_process.join(4)